1. Field of the Invention
The present disclosure generally relates to the fabrication of integrated circuits, and, more particularly, to various methods and structures of forming replacement gate structures using a gate height registration process to improve gate height uniformity and the resulting integrated circuit products.
2. Description of the Related Art
In modern integrated circuits, such as microprocessors, storage devices and the like, a very large number of circuit elements, especially transistors, are provided and operated on a restricted chip area. In integrated circuits fabricated using metal-oxide-semiconductor (MOS) technology, field effect transistors (FETs) (both NMOS and PMOS transistors) are provided that are typically operated in a switching mode. That is, these transistor devices exhibit a highly conductive state (on-state) and a high impedance state (off-state). FETs may take a variety of forms and configurations. For example, among other configurations, FETs may be either so-called planar FET devices or three-dimensional (3D) devices, such as FinFET devices.
A field effect transistor (FET), irrespective of whether an NMOS transistor or a PMOS transistor is considered, and irrespective of whether it is a planar or 3D FinFET device, typically comprises doped source and drain regions that are formed in a semiconducting substrate that are separated by a channel region. A gate insulation layer is positioned above the channel region and a conductive gate electrode is positioned above the gate insulation layer. The gate insulation layer and the gate electrode may sometimes be referred to as the gate structure for the device. By applying an appropriate voltage to the gate electrode, the channel region becomes conductive and current is allowed to flow from the source region to the drain region. In a planar FET device, the gate structure is formed above a substantially planar upper surface of the substrate. In some cases, one or more epitaxial growth processes are performed to form epi semiconductor material in recesses formed in the source/drain regions of the planar FET device. In some cases, the epi material may be formed in the source/drain regions without forming any recesses in the substrate for a planar FET device. The gate structures for such planar FET devices may be manufactured using so-called “gate-first” or “replacement gate” (gate-last) manufacturing techniques.
Immense progress has been made over recent decades with respect to increased performance and reduced feature sizes of circuit elements, such as transistors. However, the ongoing demand for enhanced functionality of electronic devices has forced semiconductor manufacturers to steadily reduce the dimensions of the circuit elements and to increase the operating speed of the circuit elements. The continuing scaling of feature sizes, however, involves great efforts in redesigning the structure of the devices and process techniques and developing new process strategies and tools so as to comply with new design rules. More specifically, to improve the operating speed of FETs, and to increase the density of FETs on an integrated circuit device, device designers have greatly reduced the physical size of FETs over the past decades. That is, the channel length of planar FETs has been significantly decreased, which has resulted in improving the switching speed of FETs. However, decreasing the channel length of a FET also decreases the distance between the source region and the drain region. In some cases, this decrease in the separation between the source and the drain makes it difficult to efficiently inhibit the electrical potential of the source region and the channel region of a planar FET device from being adversely affected by the electrical potential of the drain region. This is sometimes referred to as a so-called short channel effect, wherein the characteristic of the planar FET as an active switch is degraded.
As noted above, in contrast to a planar FET, which has a substantially planar structure, a so-called FinFET device has a three-dimensional (3D) structure. The basic features of a FinFET device include one or more vertically oriented fins that span the channel region of the device and the source/drain regions, a gate structure positioned around the exposed portions of the fins in the channel region of the device, a gate cap layer positioned above the gate electrode of the gate structure, and sidewall spacers positioned adjacent the gate structure and the gate cap layer. The sidewall spacers and gate cap layer protect the gate structure during subsequent processing operations. The gate structure may be comprised of a layer of insulating material, e.g., a layer of high-k insulating material or silicon dioxide, and one or more conductive material layers (e.g., metal and/or polysilicon) that serve as the gate electrode for the device. As noted above, the fins have a three-dimensional configuration: a height, a width and an axial length. The axial length corresponds to the direction of current travel in the device when it is operational. The portions of the fins covered by the gate structure are the channel regions of the FinFET device. In a conventional process flow, the portions of the fins that are positioned outside of the spacers, i.e., in the source/drain regions of the device, may be increased in size or even merged together by performing one or more epitaxial growth processes to form epi semiconductor material on the portions of the fins in the source/drain regions of the FinFET device. The process of increasing the size of or merging the fins in the source/drain regions of the FinFET device is performed for various reasons, e.g., to reduce the resistance of source/drain regions and/or to make it easier to establish electrical contact to the source/drain regions, etc. Even if an epi “merge” process is not performed, an epi growth process will typically be performed on the fins in the source/drain regions of the device to increase their physical size. In a FinFET device, the gate structure may enclose both sides and the upper surface of all or a portion of the fins to form a tri-gate structure so as to result in a channel having a three-dimensional structure instead of a planar structure. In some cases, an insulating cap layer, e.g., silicon nitride, is positioned at the top of the fins and the FinFET device only has a dual-gate structure (fin sidewalls only).
Thus, unlike a planar FET, in a FinFET device, a channel is formed perpendicular to the upper surface of the semiconducting substrate, thereby reducing the physical size of the FinFET device. Also, in a FinFET device, the junction capacitance at the drain region of the device is greatly reduced, which tends to significantly reduce short channel effects. When an appropriate voltage is applied to the gate electrode of a FinFET device, the surfaces (and the inner portion near the surface) of the fins, i.e., the vertically oriented sidewalls and the top upper surface of the fin (for a tri-gate device), form a surface inversion layer or a volume inversion layer that contributes to current conduction. In a FinFET device (tri-gate), the “channel-width” is estimated to be about two times (2×) the vertical fin-height plus the width of the top surface of the fin, i.e., the fin width. Multiple fins can be formed in the same foot-print as that of a planar transistor device. Accordingly, for a given plot space (or foot-print), FinFETs tend to be able to generate significantly higher drive current density than planar FET devices. Additionally, the leakage current of FinFET devices after the device is turned “OFF” is significantly reduced as compared to the leakage current of planar FETs, due to the superior gate electrostatic control of the “fin” channel on FinFET devices. In short, the 3D structure of a FinFET device is a superior structure as compared to that of a planar FET, especially in the 20 nm CMOS technology node and beyond. The gate structures for such FinFET devices may also be manufactured using so-called “gate-first” or “replacement gate” (gate-last) manufacturing techniques.
For many early device technology generations, the gate structures of most transistor elements (planar or FinFET devices) were comprised of a plurality of silicon-based materials, such as a silicon dioxide and/or silicon oxynitride gate insulation layer, in combination with a polysilicon gate electrode. However, as the channel length of aggressively scaled transistor elements has become increasingly smaller, many newer generation devices employ gate structures that contain alternative materials in an effort to avoid the short channel effects which may be associated with the use of traditional silicon-based materials in reduced channel length transistors. For example, in some aggressively scaled transistor elements, which may have channel lengths on the order of approximately 10-32 nm or less, gate structures that include a so-called high-k dielectric gate insulation layer and one or more metal layers that function as the gate electrode (HK/MG) have been implemented. Such alternative gate structures have been shown to provide significantly enhanced operational characteristics over the heretofore more traditional silicon dioxide/polysilicon gate structure configurations.
Depending on the specific overall device requirements, several different high-k materials—i.e., materials having a dielectric constant, or k-value, of approximately 10 or greater—have been used with varying degrees of success for the gate insulation layer in an HK/MG gate electrode structure. For example, in some transistor element designs, a high-k gate insulation layer may include tantalum oxide (Ta2O5), hafnium oxide (HfO2), zirconium oxide (ZrO2), titanium oxide (TiO2), aluminum oxide (Al2O3), hafnium silicates (HfSiOx) and the like. Furthermore, one or more non-polysilicon metal gate electrode materials—i.e., a metal gate structure—may be used in HK/MG configurations so as to control the work function of the transistor. These metal gate electrode materials may include, for example, one or more layers of titanium (Ti), titanium nitride (TiN), titanium-aluminum (TiAl), titanium-aluminum-carbon (TiALC), aluminum (Al), aluminum nitride (AlN), tantalum (Ta), tantalum nitride (TaN), tantalum carbide (TaC), tantalum carbonitride (TaCN), tantalum silicon nitride (TaSiN), tantalum silicide (TaSi) and the like.
One well-known processing method that has been used for forming a transistor with a high-k/metal gate structure is the so-called “gate last” or “replacement gate” technique. The replacement gate process may be used when forming planar devices or 3D devices. FIGS. 1A-1E simplistically depict one illustrative prior art method for forming an HK/MG replacement gate structure using a replacement gate technique on a planar transistor device. As shown in FIG. 1A, the process includes the formation of a basic transistor structure above a semiconductor substrate 12 in an active area defined by a shallow trench isolation structure 13. At the point of fabrication depicted in FIG. 1A, the device 10 includes a sacrificial gate insulation layer 14, a dummy or sacrificial gate electrode 15, sidewall spacers 16, a layer of insulating material 17 and source/drain regions 18 formed in the substrate 12. The various components and structures of the device 10 may be formed using a variety of different materials and by performing a variety of known techniques. For example, the sacrificial gate insulation layer 14 may be comprised of silicon dioxide, the sacrificial gate electrode 15 may be comprised of polysilicon, the sidewall spacers 16 may be comprised of silicon nitride and the layer of insulating material 17 may be comprised of silicon dioxide. The source/drain regions 18 may be comprised of implanted dopant materials (N-type dopants for NMOS devices and P-type dopants for PMOS devices) that are implanted into the substrate 12 using known masking and ion implantation techniques. Of course, those skilled in the art will recognize that there are other features of the transistor 10 that are not depicted in the drawings for purposes of clarity. For example, so-called halo implant regions are not depicted in the drawings, as well as various layers or regions of silicon/germanium that are typically found in high performance PMOS transistors. At the point of fabrication depicted in FIG. 1A, the various structures of the device 10 have been formed and a chemical mechanical polishing (CMP) process has been performed to remove any materials above the sacrificial gate electrode 15 (such as a protective cap layer (not shown) comprised of silicon nitride) so that at least the sacrificial gate electrode 15 may be removed.
As shown in FIG. 1B, one or more etching processes are performed to remove the sacrificial gate electrode 15 and the sacrificial gate insulation layer 14 to thereby define a gate cavity 20 where a replacement gate structure will subsequently be formed. Typically, the sacrificial gate insulation layer 14 is removed as part of the replacement gate technique, as depicted herein. However, the sacrificial gate insulation layer 14 may not be removed in all applications. Even in cases where the sacrificial gate insulation layer 14 is intentionally removed, there will typically be a very thin native oxide layer (not shown) that forms on the substrate 12 within the gate cavity 20.
Next, as shown in FIG. 1C, various layers of material that will constitute a replacement gate structure 30 are formed in the gate cavity 20. The materials used for the replacement gate structures 30 for NMOS and PMOS devices are typically different. For example, the replacement gate structure 30 for an NMOS device may be comprised of a high-k gate insulation layer 30A, such as hafnium oxide, having a thickness of approximately 2 nm, a first metal layer 30B (e.g., a layer of titanium nitride with a thickness of about 1-2 nm), a second metal layer 30C—a so-called work function adjusting metal layer for the NMOS device—(e.g., a layer of titanium-aluminum or titanium-aluminum-carbon with a thickness of about 5 nm), a third metal layer 30D (e.g., a layer of titanium nitride with a thickness of about 1-2 nm) and a bulk metal layer 30E, such as aluminum or tungsten.
Ultimately, as shown in FIG. 1D, one or more CMP processes are performed to remove excess portions of the gate insulation layer 30A, the first metal layer 30B, the second metal layer 30C, the third metal layer 30D and the bulk metal layer 30E positioned outside of the gate cavity 20 to thereby define the replacement gate structure 30 for an illustrative NMOS device. Typically, the replacement metal gate structure 30 for a PMOS device does not include as many metal layers as does an NMOS device. For example, the gate structure 30 for a PMOS device may only include the high-k gate insulation layer 30A, a single layer of titanium nitride—the work function adjusting metal for the PMOS device—having a thickness of about 3-4 nm, and the bulk metal layer 30E.
FIG. 1E depicts the device 10 after several process operations were performed.
First, one or more recess etching processes were performed to remove upper portions of the various materials within the cavity 20 so as to form a recess within the gate cavity 20. Then, a gate cap layer 31 was formed in the recess above the recessed gate materials. The gate cap layer 31 is typically comprised of silicon nitride and it may be formed by depositing a layer of gate cap material so as to over-fill the recess formed in the gate cavity and thereafter performing a CMP process to remove excess portions of the gate cap material layer positioned above the surface of the layer of insulating material 17. The gate cap layer 31 is formed so as to protect the underlying gate materials during subsequent processing operations.
Unfortunately, typical process flows that are performed in manufacturing gate structures using replacement gate techniques can lead to gate structures exhibiting an undesirable amount of variations in the completed gate structures. Such gate height variations can cause problems in device manufacturing, e.g., such a situation can make it more difficult to achieve and maintain planarity of subsequently deposited layers of material, it may result in conductive contacts being taller or shorter than anticipated, etc. The variations in gate height may be due to a variety of factors, e.g., the incoming silicon nitride gate cap material layer may have unacceptable thickness variations, the multiple CMP process operations that are performed in a typical replacement gate process flow can result in unacceptable levels of dishing, etc. Finding solutions to such problems is going to be even more important as device dimensions continue to shrink.
The present disclosure is directed to various methods and structures of forming replacement gate structures using a gate height registration process to improve gate height uniformity, and the resulting integrated circuit products, that may avoid, or at least reduce, the effects of one or more of the problems identified above.